Charisma may not be buyable, but it can be cultivated. Smile, look people right in the eye like they matter, focus on them and engage - people I've known who can do that comfortably are infinitely more charismatic than those who go the 100% "stage persona" route - which reeks of inauthenticity more often than not.
Image? For me it's simply trying to dress and carry myself like I belong onstage as part of a performance. By "carry myself", I mean own it honestly and comfortably.
Above all, have fun. This is SUPPOSED to be fun.
My bandmates have needed to take the time and gather the experience to get their sea legs, or stage legs in this case for charisma. The nerves and impostor syndrome may fluctuate but the charisma and comfortability can be developed as you get more confident on stage. Those first couple gigs were.....rough for the onstage banter. They, at the time, were very big on introducing each member of the band and I insisted they leave me out of that completely. They didn't get it until one time as they were doing that and ignoring me as requested someone in the crowd shouted "Who's your bassist!!" and immediately off the cuff our singer said "Oh I have no idea, we saw him at the gas station and must've just followed us here". Crowd laughs, they laugh, I feel proud of my boys, we play the song.
We have similar philosophy for image. I just try to dress on average a skosh "nicer" or more performatively than the crowd. Look like I belong on the stage as opposed to a random punter jumping up there. My bandmates in the aforementioned band will plan a little more. I guess they worry about more than one person wearing a denim vest. I don't usually do too much about that. I suppose it's easier for me since I am a bit older than them and since we play grunge influenced metal, my ratty old clothes are sorta perfect lol.